The various applications of RF quadrupole ion traps as single mass spectrometers, as tandem mass spectrometers for MS/MS examinations, as reaction vessels and measuring instruments for ionic molecule reactions, as a tool for the selective storage of ions with a uniform mass-to-charge ratio, and for the fragmentation of ions for examinations of their structure are known, for example from the standard work "Quadrupole Storage Mass Spectrometry" by R. E. March and R. J. Hughes, John Wiley & Sons, New York 1989.
Superposition of the quadrupole field of an RF quadrupole ion trap with higher multipole fields can have a favourable or adverse effect on operation, depending on the operating mode and operating phase of the ion trap.
Superposition of relatively weak, higher multipole fields of the same frequency on the RF quadrupole field has considerable effect on the stored ions if these ions stay, due to the amplitude of their secular oscillations, not only at the center of the quadrupole field but also in the noncentral regions of the ion trap. This can occur if (a) the ions are introduced into the ion trap from outside, if (b) the secular oscillation of the ions is excited by additional electrical fields (for example, with collision-induced fragmentation of the ions) or if (c) the ions are mass-selectively ejected from the trap for analysis.
The generation of quadrupole fields with superposed weak multipole fields of even ordinal numbers by the special shape of the electrodes is known from U.S. Pat. No. 5,028,777. Superposition with weak hexapole and octopole fields is described in U.S. Pat. No. 5,170,054.
Higher multipole fields with an even ordinal number (octopole fields, dodecapole fields) stabilize the storage of ions against interference fields, the frequency of which is in resonance with the secular oscillation (J. Franzen, Int. J. Mass Spectrom. Ion Proc., 106 (1991) 63). The interference fields can be generated by interference frequencies on the storage RF voltage, by interference frequencies on other electrodes, or also by nonlinear resonance conditions originating from the chance or desired deviation in the shape of the electrodes. If resonance with an interference frequency arises, the ion oscillation absorbs energy and the oscillation amplitude increases. The multipole fields cause a change in the secular oscillation frequency of the ions with growing oscillation amplitude. As a result, on enlargement of their oscillation due to resonant energy absorption, ions quickly fall out of step with the exciting interference frequency, and thus fall out of resonance. Further absorption of energy does not take place. In contrast to a pure quadrupole field, there is consequently a distinct voltage threshold for the removal of ions from the trap in the case of multipole field superposition.
One use of ion traps in mass spectrometric measurement is based on the mass selective ejection of ions from the trap. There are different resonance phenomena which can be used for this: resonance at the edge of the stable storage area (U.S. Pat. No. 4,548,884), resonance with an electrically generated dipole field (U.S. Re 34 000), and resonance with a nonlinear resonance condition by superposition with higher multipole fields (EP 0 383 961 A1). In all cases, ejection enables the ions of successive masses (mass-to-charge ratios to be more precise) to be sequentially detected outside the ion trap with an ion detector and measured as an ion stream. Even multipole fields can favourably influence this mass selective ejection of ions from ion traps by increasing the mass resolving power. Here too, the multipole-generated dependence of the oscillation frequency on the amplitude plays a role. Enlargement of the amplitude on convergence with the resonance condition for ion ejection enables the oscillation frequency to be changed in such a way that resonance is reached more quickly and the absorption of energy for amplitude enlargement is intensified.
Uneven higher multipole fields (hexapole fields, decapole fields), which are superposed in addition to the even multipole fields, can again improve mass selective ion ejection by determining the direction of ejection in such a way that the ions always leave the trap through the same end cap. The other end cap is not even reached by the oscillating ions.
The superposed multipole fields are, however, disadvantageous for mass selective storage of ions in the ion trap. For this type of storage, all undesired ions are prevented from storage by a mixture of alternating voltages with differing frequencies which is additionally applied to the electrodes. The secular frequencies of the undesired ions are constantly excited, causing them soon to leave the ion trap. The frequency mixture does not, however, include the frequency for excitation of the desired ion type, enabling it to be caught and stored in the ion trap by being slowed down in a collision gas. The simultaneous storage of more than one type of ion is also possible by leaving more than one gap in the frequency mixture. These can be ions which are generated outside the ion trap and introduced into it by ion-optical means or, however, ions which are generated within the ion trap by any particular type of ionization process. Patent application DE-P 43 16 737.3 describes one particular digital generation method for the frequency mixture, which is used for selective ion storage, as well as quoting further literature.
This type of mass selective storage of ions by one or more frequency gaps in an applied frequency mixture requires, however, that the ions have a secular oscillation frequency which is independent of their oscillation amplitude. The ions can only then be stored with good mass resolution if their oscillation frequency is independent of the oscillation amplitude since only then are they spared excitation by the frequency mixture in the narrow frequency gap. The oscillation frequency is, however, independent of the oscillation amplitude only in a pure quadrupole field. Consequently, selective storage of individual ion types with good mass resolution is possible only in a pure quadrupole field.
So far only arrangements for ion traps have been described which generate either a relatively pure quadrupole field or, a more or less intense superposition of a quadrupole field with higher multipole fields. The type of field is predetermined by a fixed electrode structure. Narrow correcting rings between the end cap electrodes on the one hand and ring electrodes on the other serve only for correction of the quadrupole field.
Therefore, it is among the objects of the invention to produce an RF quadrupole ion trap, which on the one hand is able selectively to store selected ions with good mass resolution, and on the other hand to store ions in a stable manner and eject them selectively with good mass resolution.